In 3GPP-LTE (3rd Generation Partnership Project Radio Access Network Long Term Evolution, hereinafter, referred to as “LTE”) uplink, periodic sounding reference signals (P-SRS) are used as reference signals (SRS (sounding reference signal)) to measure uplink receiving quality.
Furthermore, in LTE, an SRS transmission subframe (hereinafter, referred to as “common SRS subframe”) which is common to all terminal apparatuses (hereinafter, simply referred to as “terminal” or also referred to as “UE (User Equipment)”) is configured. This common SRS subframe is defined by a combination of a predetermined periodicity and a subframe offset on a per-cell basis. In addition, the information on the common SRS subframe is broadcasted to terminals within the cell. For example, when the periodicity is equal to 10 subframes and the offset is 3, the third subframe in a frame (consisting of 10 subframes) is configured as a common SRS subframe. In a common SRS subframe, all the terminals within the cell stop transmission of data signals in the last symbol of the subframe and use the period as the resources for SRS transmission (reference signals) (hereinafter referred to as “SRS resources”).
Meanwhile, subframes for SRS transmissions are individually configured for terminals by a higher layer (i.e., RRC layer higher than the physical layer) (hereinafter, referred to as individual SRS subframe). Each terminal transmits an SRS in the configured individual SRS subframe. In addition, parameters for SRS resources (hereinafter, may be referred to as “SRS resource parameters”) are configured and indicated to each terminal. The SRS resource parameters include the bandwidth, bandwidth position (or SRS bandwidth starting position), cyclic shift and comb (corresponding to identification information on the subcarrier group) of the SRS, for example. The terminal transmits an SRS using the resources specified by the indicated parameters. Additionally, SRS frequency-hopping may be configured.
Next, conventional (LTE Rel.10) SRS transmission power control will be described.
Transmission power PSRS,c(i) of an SRS in subframe #i of serving cell #c is calculated according to following equation 1 as described in NPL 1. The serving cell is a cell that indicates control information to a terminal in communication.[1]PSRS,c(i)=min{PCMAX,c(i),PSRS_OFFSET,c(m)+10 log10(MSRS,c)+PO_PUSCH,c(j)+αc(j)·PLc+fc(i)}   (Equation 1)
In equation 1, PCMAX,c [dBm] represents maximum transmission power of SRS that can be transmitted by a terminal, PSRS_OFFSET,c(m) [dB] represents an offset value of transmission power of SRS with respect to transmission power of PUSCH transmitted by the terminal (parameter set from a base station apparatus (hereinafter, may be simply referred to as “base station” or may also be referred to as “eNB”)), MSRS,c represents the number of frequency resource blocks assigned to SRS, PO_PUSCH,c(j) [dBm] represents an initial value of transmission power of PUSCH (parameter set from the base station), PLc represents a path loss level [dB] measured by the terminal, αc(j) represents a weighting factor representing a compensation ratio of the path loss (PLc) (parameter set from the base station), and fc(i) represents a cumulative value in subframe #i including past values of TPC (transmission power control) command (control value, for example, +3 dB, +1 dB, 0 dB, −1 dB) subject to closed loop control. In PSRS_OFFSET,c(M), values are set for m=0, 1 respectively. To be more specific, a parameter value of m=0 is used in the case of Type 0 SRS (also referred to as “P-SRS”) in PSRS_OFFSET,c(M) or a parameter value of m=1 is used in the case of Type 1 SRS (also referred to as “aperiodic SRS (A-SRS)”). PSRS_OFFSET,c(M) is expressed in step widths of 1.5 [dB] within a setting range of −10.5 [dB] to 12.0 [dB]. That is, PSRS_OFFSET,c(m) is expressed by 4 bits. Furthermore, values are set in PO_PUSCH,c(j) and αc(j) for j=0, 1, 2, respectively.
Here, the path loss (PLc) is a value measured by a terminal using reference signals transmitted by a serving cell of the terminal and is calculated according to following equation 2.[2]PLc=referenceSignalPower−RSRP  (Equation 2)
In equation 2, referenceSignalPower represents a transmission power value of a reference signal of a serving cell indicated from the serving cell, and RSRP (reference signal received power) represents receiving power of a reference signal calculated by a terminal using a filter coefficient (averaged length) indicated from the serving cell.
In the uplink of LTE-Advanced, which is an evolved version of LTE, aperiodic SRS (hereinafter referred to as “A-SRS”) is used in addition to P-SRS introduced from LTE. This A-SRS transmission timing is controlled by trigger information (e.g., 1-bit information). The trigger information is transmitted from the base station to the terminal using a control channel of the physical layer (that is, PDCCH). That is, the terminal transmits A-SRS only when A-SRS transmission is requested by trigger information (that is, A-SRS transmission request). Studies are underway to assume the A-SRS transmission timing to be a first common SRS subframe four subframes after the subframe in which the trigger information is transmitted. As described above, P-SRS is periodically transmitted, while A-SRS can be transmitted to the terminal for a short period in a concentrated manner only when uplink transmission data is generated in bursts.
In LTE-Advanced, studies are being carried out on a heterogeneous network (HetNet) using a plurality of base stations having coverage areas of different sizes. Furthermore, in LTE-Advanced, studies are underway to apply CoMP (coordinated multiple point transmission and reception), which is a communication scheme in which a plurality of cells (base stations) cooperate to transmit data to a terminal for the purpose of improving mainly the throughput of a user located on a cell edge in a heterogeneous network.
The heterogeneous network is a network jointly using a macro base station that covers a large coverage area (hereinafter, may also be referred to as “macro cell” or “macro eNB” or “HPN (high power node)”) and a pico base station that covers a small coverage area (hereinafter, may also be referred to as “pico cell” or “pico eNB” or “LPN (low power node)”). For example, in the operation of a heterogeneous network, a pico eNB having small transmission power is installed in a coverage area of a macro eNB having large transmission power, and the macro eNB and the pico eNB are connected together using a cable (an optical fiber or the like). Studies are underway to apply downlink CoMP (downlink CoMP) in which the pico eNB and the macro eNB cooperatively transmit data signals to a pico terminal (pico UE, terminal controlled by the pico eNB) in such a heterogeneous network environment (see FIG. 1).
In a system using CoMP, studies are underway to perform CoMP control in accordance with a propagation path condition such as selection of an optimum transmission point or selection of a transmission weight from among a plurality of cells (base stations) using reference signals (e.g., P-SRS and A-SRS) for measuring uplink channel quality from the terminal to the base station (e.g., see NPL 2).
A plurality of base stations such as a macro eNB and pico eNB receive SRS transmitted from a terminal and measure channel quality (e.g., SINR) (see FIG. 2). SINR measured values of the base stations (that is, propagation path conditions between the base stations and the terminal) are compared and an optimum transmission point in downlink CoMP is thereby determined.
When CoMP is applied in a TDD (time division duplex) system, a plurality of base stations can estimate downlink channel responses from uplink channel responses (channel quality) at the respective base stations calculated from SRS using reversibility of the propagation path. In this case, estimate values of downlink channel responses in the respective base stations are compared, and an optimum transmission point and transmission weight in downlink CoMP are determined.
In a system that applies CoMP in a heterogeneous network environment, a pico UE is located within the coverage area of a macro eNB (can receive transmission signals of the macro eNB). For this reason, not only a pico eNB but also a macro eNB can be selected as a transmission point for the pico UE. That is, when channel quality between the pico UE and the macro eNB is good, it is possible to improve the downlink throughput performance by transmitting data from the macro eNB which is not the serving cell of the pico UE to the pico UE in a coordinated manner.